Seismic detectors called geophones are well known in the art. The geophone is basically a seismic-to-electrical transducer, and, when used, it is typically implanted in the surface of the earth. When a acoustic disturbance is generated at a location away from the place where the geophone is implanted, it propagates into the ground and the reflected disturbance is collected by the geophone for use in the analysis of the underground structure. In this case, the geophone basically detects the motion of the ground surface, i.e., the acceleration, the velocity or the displacement of a point of the ground surface. Although there are several variations, the geophone is mainly comprised of a moving mass, a spring for suspending the mass and a case for housing therein the moving mass suspended by the spring. With this structure, when the case is set in motion, for example, due to reception of the reflected disturbance, there is produced a relative motion between the moving mass and the case through the spring and such a relative motion is utilized to produce a variable, such as a voltage, for the measurement of the acceleration, the velocity or the displacement of a point of interest.
On the other hand, a servo accelerometer has recently come to draw much attention. The servo accelerometer is basically an accelerometer with a feedback circuit and thus it defines a closed-loop detector system. The accelerometer has been used for measuring the vibration of machine tools and buildings, and it is mainly comprised of a moving mass for detecting the acceleration and a spring for supporting the moving mass. In such accelerometer, the force applied to the moving mass due to the acceleration is electrically detected for measurement of the acceleration. Such accelerometer usually defines an open-loop detector system. Thus, the acceleration-responsive geophone may be considered as a kind of accelerometer.
The servo accelerometer is an improvement over the above-described normal accelerometer and it is so structured to measure the acceleration from an electric current which is required to produce a force or torque to bring the moving mass back to the original position. Thus, the servo accelerometer is significantly enhanced in accuracy as well as stability as compared with the normal open-loop type accelerometer. One of the remarkable features of the servo accelerometer is the static acceleration sensitivity, which allows the servo accelerometer to be used for the measurement of the angle or orientation, indicating the possibility for the application in the attitude control.
As described above, the servo accelerometer is far superior to the open-loop type accelerometer in many respects, but still it needs to be further improved so as to allow wider and more extensive applications.